An SRAM (Static Random Access Memory) is a type of RAM which allows writing and reading operations at any time without a refreshing operation when the electricity is on; and, it has a flip flop circuit that is capable of storing 1 bit of information and two information transfer MISFETs (Metal Insulator Semiconductor Field Effect Transistors). The flip flop circuit has, for example, a pair of drive MISFETs and a pair of load MISFETS.
In such a memory cell, soft errors due to, for example, α rays become a problem. This phenomenon occurs when α rays contained in the cosmic rays from outer space or alpha rays emitted from a radioactive atom contained in the packaging material of an LSI damage the information stored in the memory cell. Addition of a capacitance to the information storage portion (input/output portions of the flip flop circuit) in the memory cell to increase the capacitance of the information storage portion is under investigation as a countermeasure against this α ray problem.
There is a potential danger that, as scaling-down of the memory cell proceeds and the operating voltage lowers, the capacitance which the memory cell itself possesses decreases and soft errors which damage the information tend to occur.
For example, there is a technique (refer to, for example, the below-listed Patent Document 1) of forming a capacitor between two interconnects for cross-coupling input/output terminals of a flip flop circuit for storing information, by adding two metal film layers, which will constitute capacitor electrodes, and a thin insulating film interposed therebetween, to increase the capacitance of a storage node of the memory cell, thereby preventing lowering of the soft error resistance. Patent Document: Japanese Patent Application Laid-Open No. 36252/1997